1. Field of the Invention
The present invention relates to controlling a switching power converter in more than one operation mode to increase the efficiency of the power converter at light load conditions.
2. Description of the Related Arts
Efficiency of a switching power converter is governed largely by two types of loss: switching loss and conduction loss. In most switching power converters, switching loss plays a greater role as the load across the output of the power converter is decreased. Conversely, conduction loss plays a greater role as the load is increased. In order to reduce loss at both high load conditions and light load conditions, some conventional switching power converters use pulse-width-modulation (PWM) and pulse-frequency-modulation (PFM) at different load conditions. PWM mode is generally more efficient than PFM mode at higher load conditions. PFM mode is generally more efficient than PWM mode at lower load conditions.
In PWM mode, a controller of the power converter turns on the switch of the switching power converter at a constant switching frequency but varies the duty cycle of the switch by adjusting how long the switch remains on during each switching period. Conversely, in PFM mode, the switch is turned on for a set duration, but the switching frequency is varied depending on the load. Specifically, in PFM mode, the switching frequency is increased as the load increases, and switching frequency is decreased as the load decreases.
Some conventional power converters operate in both PFM mode and PWM mode depending on the load conditions to achieve high efficiency at varying load conditions. Figure (FIG.) 1 is a graph illustrating the operation of such conventional power converter in constant voltage (CV) mode represented by line J-K. Line K-L represents operation of the power converter in constant current (CC) mode. In CV mode, the output voltage Vout remains constant. Therefore, the load is proportional to the output current Iout. As illustrated in FIG. 1, the power converter operates in PWM mode during high load conditions between I3 and I4. In light load conditions (output current below a threshold output current I3), the power converter operates in PFM mode. The threshold level I3 is generally set below 10% of the maximum current (or load) that can be handled by the power converter.
Such low setting of threshold level I3, however, does not help improve the efficiency at 25% load. Therefore, conventional PWM/PFM control with low threshold level I3 does not help improve the overall average efficiency, for example, as defined by world wide energy standards such as US-EPA 2.0 and EU-CoC. Such standards specify the average efficiency of the power converter based on averaging of the efficiencies at four loading points: 25% load, 50% load, 75% load, and 100% load. Therefore, it is desirable to transition to PFM mode at a higher loading condition (e.g., 50% of the maximum current) to take advantage of power saving feature of PFM mode.
With conventional control approaches, however, PWM mode to PFM mode transition at higher load conditions suffer from serious performance problems such as excessive voltage ripples caused by unsmooth transition between PWM mode and PFM mode.
Other conventional power converters operate in PFM mode throughout the entire range of load conditions. The power converters operating only in PFM mode suffer from audible noises when the switching frequency drops to around 16 kHz. With the wide dynamic range of possible loads, it is difficult to stay out of the audio band using this technique.
Some conventional power converters operate in PWM mode throughout the entire range of load conditions. Such conventional power converters operate at a switching frequency above the audible range, and therefore, do not generate audible noises. The power converters operating only in PWM mode, however, suffer from low efficiency in light load conditions.
In addition to the active-mode efficiencies where a power converter provides an output power that is a fraction of the maximum output intended for the power converter, power consumption associated with no-load conditions is another regulation target of the world wide energy standards. A no-load condition refers to a power convert operation condition during which the input of the power converter is connected to a power source, but the output is not connected to any load. At the no-load conditions, although its output power and load is zero, the power converter still consumes certain input power mostly due to the switching and conduction losses caused by the switch, the power consumed by the controller itself, and the power consumed by resistors and capacitors inside of the power converter.